Commercial dyeing of textiles or fabrics usually consists of immersing the fabric in a dye bath containing an appropriate solution of the dyestuff until the desired shade is reached through absorption of the dye onto the fabric. As there are numerous variables that can alter the efficiency of absorption of the dye it is conventional to add more dyestuff to the dye bath than is required and to control the extent or depth of dyeing by time. The consequence of this approach is that further steps are required to remove excess unfixed dye from the fabric after dyeing has been completed. Without the removal of the excess dye problems such as dye run or transfer from the finished article may occur. In addition, it may lead to downstream contamination of the processing plant.
Dyeing of polyester fabric is not particularly easy as the nature of the fibres of the component polymer is such that they are extremely hydrophobic. In addition, the fibres are not dissolved or degraded by organic solvents. In order to achieve effective dyeing of polyester fabric it has been necessary to use specialist dyestuffs and harsh conditions in specialised equipment.
Polyester fabric is typically dyed using disperse dyes. Disperse dyes are ready made dyes that do not have any ionic character and as such they are insoluble or only poorly soluble in water under ambient conditions. Such dyes are utilised in the dyeing process by dispersing them in acidified water at elevated temperatures, e.g. 80° C. to 100° C., or at both elevated temperature and pressure, e.g. 105° C. to 140° C. and 1.1 to 3.6 bar. Such conditions result in the dye diffusing into the plasticized polyester fibres to form a molecular dispersion in the polymer matrix.
Dispersing agents and carrier chemicals are commonly used in the dyeing of polyester fabric. Dispersing agents are needed to keep the bulk of the poorly soluble disperse dye in an homogeneous state throughout the dyebath liquor. Such dispersing agents are typically strong surfactants such as alkylsulphonates and alkylarylsulphonates. Carrier chemicals are substances that are designed to swell the fibres and help facilitate the diffusion of the disperse dye into the fabric. Typical carrier chemicals that may be used are based on glycol ethers.
Once sufficient colour strength has been achieved on the fabric it is necessary to remove excess dye. As the dyeing of polyester fabric with disperse dyes is a diffusion process there will be a certain accumulation of dye adsorbed onto the surface of the fabric which has not diffused into the bulk of it. This accumulation of dye will have a more tenuous attachment to the fabric resulting in problems in the finished cloth, such as undermining the shade of the fabric. It may also affect the washing and rubbing fastness of the dye.
The process of removing excess dye that serves to eliminate these problems is called reduction clearing. Reduction clearing usually uses a strong reducing agent at a high temperature and pH to remove excess dye. The most commonly used reagent in reduction clearing is sodium dithionite in the presence of sodium hydroxide. This reagent has several disadvantages, not least its reactivity which leads to complications in handling. It is unstable in non-alkaline conditions where decomposition will occur, even resulting in spontaneous combustion. Another disadvantage of using sodium dithionite is that it acts as a sulphonating agent that can act on any residual surfactants remaining in the fabric. This can result in any residual surfactants becoming persistent. Use of such sulphur containing compounds also suffers from the disadvantage that as the compounds are not readily biodegradable the waste water from the process needs to be extensively treated before it can be returned to watercourses.
U.S. Pat. No. 6,730,132 discloses a process for reduction clearing of polyester textiles that comprises adding to the acidic dyeing liquor or wash bath an after-treatment composition comprising dithionite/acid acceptor sulphinate optionally mixed with sulphonate.
It has now been found that by adding a weak organic acid or a salt thereof, such as ascorbic acid or citric acid, or a salt thereof, as the reduction clearing agent to the dyed polyester fabric, preferably following the removal of dyeing liquor, heating for a period of time and subsequently removing the liquid, excess dye is readily removed from the dyed polyester fabric without the need to use sulphur-containing compounds.